The present invention relates generally to apparatus and methods for preventing two or more plugs in a plug container from being released from the container into a well except in a predetermined sequence. The present invention relates more particularly, but not by way of limitation, to an apparatus and a method for controlling the use of a plug container connected to a well during a cementing job, which plug container includes two stacked plugs and two fluid inlet valves which are to be sequenced to the release of the two plugs.
During cementing of wells, a bottom plug is inserted into the casing ahead of the cement slurry and is pumped down the casing. A top plug is then inserted in the casing on top of the cement. The top plug separates the cement from the drilling mud or fluid which is used to force the cement out through the bottom of the casing and up through the annular space between the hole and the casing. It is important that the plugs be released at the proper time during the cementing process so that they separate the cement slurry from the drilling mud or fluid. Cementing plugs are usually stored in a plug container on top of the casing at the well head. A bar or other means supports a plug in the plug container until the appropriate time for releasing the plug. The bar is then removed, thereby allowing the plug to drop into the casing. Various levers and rods have been proposed for temporarily retaining a plug in the plug container.
A particular embodiment of a plug container includes two plugs, held by two plug release plungers, and three fluid inlets, connected through manifold valves to a manifold through which the cement and drilling mud or fluids are pumped. The plug release plungers and the manifold valves must be operated to release the plugs and admit the fluids at the proper times. They can be manually operated by an operator at the plug container, or more preferably they can be operated remotely from the well head where the container is attached. Improved safety, enhanced convenience, and automated control "on the fly" are some of the reasons why remote control is preferred.
In either local or remote control, the operator could release an upper plug before a lower plug has been released and the operator could open a fluid inlet valve before a lower plug has been dropped. Either of these situations can be hazardous or can cause an improper cementing job to result. Therefore, there is the need for an automatic sequencing controller which prevents an operator from releasing the wrong plug or opening the wrong inlet valve during the cementing job. Although there is the need for such sequence control, there is also the need to permit the closure of any manifold valve at any time so that they can be shut down in an emergency, for example. Opening and closing the manifold valves in any order once a cementing job has been completed is also desirable so that the fluid inlet system can be readily cleaned, for example.